This application is based on and incorporates herein by reference Japanese Patent Application No. 2000-127556 filed on Apr. 27, 2000.
1. Field of the Invention
The present invention relates to a guide representation of branch roads, and particularly to a technique for providing an appropriate guide representation concerning a junction having a loop exit road.
2. Related Art
A navigation system is known as equipment for enabling a driver to arrive at the destination smoothly. It detects the present position of a motor vehicle by GPS (Global Positioning System) or the like as the motor vehicle travels, so that it represents the present position with a road map on a display. Moreover, some navigation systems calculate an appropriate travel route from the present position to the destination and provide the calculated result as a guide route to help a driver to travel more smoothly. In this case, a conventional map display device usually provides guidance by adding the guide route from the present position to the destination on a map showing the surrounding areas of the present position of the motor vehicle. This method displays the guide route in a color different from that of other roads so that the driver may readily recognize it. Furthermore, enlarged views of junctions on the guide route where the driver should turn are displayed so that the driver can accurately recognize the junction where he/she should turn next.
However, in case of providing guidance concerning junctions on roads which have the attributes of an expressway or a freeway, it may be difficult for a driver to decide about the turn direction, because it appears as if roads overlap each other on an enlarged view by reason of very small forking angle of the roads. Furthermore, in case of providing guidance concerning a loop connection road (ramp) which connects an underpass (ground road) to an overpass (elevated road) at a solid crossing, the following problem arises.
A technique for enabling a driver to easily know the direction in which his/her motor vehicle should be driven is proposed. It represents not only the turn direction at the next junction but also the turn direction at the junction beyond next. Actually, when the motor vehicle is directed to travel along the arrow on a cloverleaf-type ramp way as shown in FIG. 15A, that is, turn to the right at a first guide point G1 and thereafter go straight on without turning to the right at a second guide point G2, it displays a schematic diagram that represents two successive simple junctions as shown in FIG. 15B. However, since the junction actually has a loop exit road, it is not consistent with the displayed diagram. Thus it does not necessarily provide intelligible guidance.
Therefore, in the case of providing guidance concerning such a junction, other techniques should be employed. For example, JP-A-5-10775 proposes to display the turn direction and the name of the place to which the exit road from the junction ultimately leads at the first turning point where a motor vehicle should turn at the junction by using junction information (the shape of the junction, the number of lanes, a solid crossing or a grade crossing, etc.) received from a beacon when the next junction is a solid crossing. However, this technique cannot be applied to a junction on which a beacon is not installed since it uses the junction information from the beacon. Moreover, since this technique displays only the turn direction and the name of the place to which the exit road from the junction ultimately leads and does not display the shape of the junction, a driver cannot know that the exit road from the junction is a loop when the guide route runs along a cloverleaf-type ramp way as shown in FIG. 15A as described above.
In contrast to this, JP-A-61-213720 proposes to store information representing that a junction has a complex shape as flag data beforehand by incorporating the data into map data. When this provides guidance concerning an interchange of an expressway or a road having a complex shape, it retrieves the detailed shape data or the schematic diagram of a complex junction from image memory based on the flag data and displays that when it provides guidance immediately before the junction. In this case, it is not required to receive the information from the beacon. Furthermore it can represent that the exit road from the junction is a loop.
However, in the case of employing this technique, it is required to determine whether the actual shape of each junction fulfills the condition for a complex shape by field surveys beforehand, and to generate data of or assign a schematic diagram to the detailed shape of each junction that fulfills the condition for a complex shape and store them in the image memory. That is, it is required to investigate a large number of junctions (branch roads) in the area to which the technique is applied (for example, all parts of a country) one by one, and to collect data of the junctions that fulfill the condition for a complex shape. Thus, it actually requires a great deal of time and cost for data preparation.
The present invention has therefore an object to provide an appropriate guide representation concerning a looped branch road without consuming much time and costs.
The junction guide device according to the present invention stores a guide route from a point of departure to a destination and provides guidance for a driver while a motor vehicle travels along the guide route. When the motor vehicle approaches a junction concerning which the guidance should be provided, the device automatically determines by using stored road data whether the guide route forks to a loop road at the junction, and displays an appropriate schematic diagram of the junction. Therefore it is not necessary to make field surveys for determining whether each of actual junctions forks to a loop road one by one. The displayed schematic diagram represents the forking direction of the guide route at the junction. If it is determined that the guide route forks to a loop road, the displayed schematic diagram also includes a representation of the loop road. Thereby the driver can know the forking direction of the guide route at the next junction and whether the guide route forks to a loop road at the junction.
The displayed schematic diagram is one of the stored schematic diagrams each of which represents a common pattern found in the actual junctions. Since these common patterns can be easily obtained from administrative offices that have jurisdiction over road construction or literature, it is not necessary to make field surveys for obtaining the pattern of each of the actual junctions one by one. Further it is not necessary to install an information source such as a beacon along roads. Therefore the device can provide an appropriate guide representation concerning a looped branch road without consuming much time and costs, though the provided representation slightly differs from the actual shape of the loop road.